In the early years of the nuclear industry there was this energetic burst of ideas of how to develop nuclear reactors to produce heat to create steam to generate electricity. A heat source is all that was needed, be it coal, O&G or nuclear fission. Because the nuclear industry was so new, there was some confusion about which reactor type to use. There were basically three different design strategies that would produce heat needed to generate electricity.
Water cooled solid uranium fueled reactors; were originally designed by the US Navy for their ships and were very small and efficient for use on remote ships at sea where there was plenty of water for cooling purposes. When the AEC (Atomic Energy Commission) decided to upscale these water cooled reactors for land use, several obstacles had to be overcome. The industry overcame all those at great expense and now there are almost 500 reactors world wide producing trillions of megawatts of clean, safe electricity and spent nuclear fuel.
Molten salt thorium fueled reactor; was also designed and prototyped for 6 years as the commercial solution to nuclear powered energy during that same period. Unfortunately, through misunderstanding by non-scientific politicians, there was a lot of confusion about switching to a thorium based fuel system. Thorium molten salt reactors could have been any size, used any where, without any safety concerns, and did not produce a waste stream for proliferation. Perfect solution, lacked funding.
Molten salt cooled solid fuel fast breeder reactor; was originally designed and prototyped over a 30 year period as a clean commercial solution that used existing spent nuclear fuel reprocessed and did not produce more waste. It actually breed its own fuel but this program was also defunded by a very confused President and because of competition from the established water cooled nuclear industry.
Now the industry is at a crossroad again between existing designs or switching to more financially efficient reactors. At the same time the anti-nuclear movement is continuing their war on nuclear even with all the proof that nuclear is safer, cleaner, more reliable, more efficient, and in the long run more cost effective than all other sources to power the world.